For loose thread, stem or washer problem, here is a guide on how to fix the issue.
Replace the protective cap over it. Take the washer and stem to a local plumbing supply store to obtain the correct replacement parts. If you turned the water off to the entire house, and there are faucets at the lower portion of the home, turn them on as well to completely remove the water from the pipes.
When the screw is removed, pull off the handle. There is no need to turn off the water supply for the entire home unless the faucet is not connected to any shut-off valve.
As soon as the locknut is removed, the stem can also be pulled out of the faucet. Make sure no one is using the water supply during repairs.
Locate the screw and loosen it with a screwdriver. Locate the shut-off the valve located under the sink, behind the faucet assembly or behind a panel. Locate the rubber washer at the bottom of the stem. If the stem is to be replaced, the washer has to be replaced as well to ensure that the faucet does not leak due to a faulty or worn out washer.
Use a small screwdriver to loosen the screw. Install the washer to the stem. Turn on the faucet and let the water drain.
Tighten it afterward with a wrench, but not too much so as to avoid causing damage. If the screw is corroded, purchase a similar screw for replacement.
Position it into the faucet assembly and hand-tighten it first to ensure the threads are aligned correctly. To ensure that the screw does not get corroded easily, clean any debris from the screw hole before inserting the screw. Remove the washer. Some stems are threaded to the faucet assembly, so in order to repair the leak, the stem has to be removed to inspect it for possible damage. Every handle is secured to the faucet with a screw.
Use a crescent wrench to remove the locknut fastening the stem in place. Install the handle back over the stem and tighten the screw. If the faucet stem is threaded to the assembly, turn it counterclockwise with a wrench until it disengages.
Stem-type faucets usually leak due to issues in the faucet stem. If the screw is hidden by a cap, pry off the cap using the flat edge of the screwdriver.
If the screw is corroded and hard to remove, apply some penetrating oil and wait for a few minutes. To ensure no water drips off to the floor during repairs, drain the water from the water supply lines.